1. Field
Embodiments of the present invention relate to energy storage systems. In particular, high-voltage flywheel energy storage systems store electrical energy as kinetic energy in a rotating flywheel. The stored energy can also be released from the high-voltage flywheel energy storage system.
2. Background
Large-scale energy storage has the potential to help modernize electrical power distribution. Energy storage can help manage intermittent renewable energy generation, electricity load shifting, black-start capabilities, electricity price fluctuations, and back-up power supply.
There are currently several large-scale energy storage technologies that attempt to address these modernization challenges facing the energy storage industry, including: advanced batteries; electrochemical capacitors (EC); pumped hydro; compressed air; and flywheel energy storage systems.
Due to low costs associated with lead acid batteries, they have been a popular choice for power quality and uninterruptable power supply (UPS) applications. However, the effectiveness of lead acid batteries for large-scale applications is limited by a short battery life cycle and a variable discharge rate. Li-ion batteries are often seen as an alternative or replacement for lead acid due to a longer life cycle. The effectiveness of Li-ion batteries for large scale energy storage is limited, however, by a high manufacturing cost and by security concerns associated with large-scale implementation. Metal-Air batteries are the most compact and potentially the least expensive battery to manufacture. However, the effectiveness of Metal-Air batteries is limited by a very short life cycle and low efficiency (e.g., approximately 50%). Sodium-sulphur (NaS) battery technology has shown promise as a solution for large-scale implementation. NaS batteries have high energy density but require high operating temperatures and have a relatively short life span. Battery technologies typically have an average AC to AC round-trip efficiency of approximately 64%. And, electrochemical battery technologies generally have a usable life that is degraded by the number of charge/discharge cycles.
Electrochemical capacitors (EC) are energy storage devices that have longer life cycles and are more powerful than lead-acid batteries. However, it is not feasible to implement ECs on large-scale projects due to their high cost and low energy density.
Conventional pumped hydro as an energy storage technology uses two water reservoirs that are separated vertically. An energy potential due to gravity is associated with the energy of the water travelling from the elevation of higher potential energy to the elevation of lower potential energy. During off-peak hours, electrical power is used to pump water from the lower reservoir to the upper reservoir. As demand for electrical energy increases, the water flow is reversed to generate electricity. Pumped hydro offers beneficial energy management and frequency regulation, but requires unique site requirements and large upfront capital costs.
Compressed air energy storage (CAES) uses a combination of compressed air and natural gas. A motor pushes compressed air into an underground cavern at off-peak times. During on-peak times, compressed air is used in combination with gas to power a turbine power plant. A CAES uses roughly 40% as much gas as a natural gas power plant and similarly to pumped hydro, requires unique site requirements and large upfront capital costs.
Flywheel energy storage systems have emerged as an alternative to the above-identified energy storage technologies. Flywheel energy storage systems are currently used in two primary commercial applications: UPS and power frequency regulation (FR). Both UPS and FR require extremely quick charge and discharge times that are measured in seconds and fractions of seconds. Flywheel technologies have high reliability, long service life, extremely low maintenance costs, high power capability, and environmental friendliness. Flywheel energy storage systems store energy in a rotating flywheel that is supported by a low friction bearing system inside a housing. A connected motor/generator accelerates the flywheel for storing inputted electrical energy, and decelerates the flywheel for retrieving this energy. Power electronics maintain the flow of energy into and out of the system to mitigate power interruptions, or alternatively, manage peak loads.
Often, the rotating flywheel and motor/generator rotor operate in at least a partial vacuum to reduce windage losses due to drag forces acting on the flywheel. In the case of high-voltage flywheel energy storage systems, power supplies present a raised risk of ionization and plasma formation on the windings in the motor/generator. Such plasma formation can lead to electric arc discharge. This is especially true when the motor/generator operates in a partial vacuum environment.